Differential compaction mechanism for earth fissures near Casa Grande, Arizona

Precise gravity measurements indicate that earth fissures or tension cracks caused by ground-water withdrawal within a 10-km² area southeast of Casa Grande, Arizona, are associated with relief on the buried interface between the alluvial aquifer and underlying bedrock. All of the fissure zones; which have a cumulative length of >8.7 km, occur above either ridges or steps in the bedrock surface. Intersecting fissure zones overlie intersecting bedrock features, and the angle of intersection of the zones accurately reflects the angle between the bedrock features. These relations suggest that the fissures are forming in response to localized differential compaction caused by localized variations of aquifer-system thickness. Topographic profiles across fissures on undisturbed desert floor confirm differential compaction proportional to the variations in aquifer thickness. The occurrence of the fissures at points of maximum convex-upward curvature in profiles of both the topographic and buried bedrock surfaces indicates that the fissures result from tensile strains caused by bending of the strata above the buried bedrock features in response to the differential compaction. Tensile strains at failure are estimated to range from ∼ 0.02% to 0.2% on the basis of modeling of the bending process.